1. Field of the Invention
This invention relates to a device for the controlled release of biologically active substances and other chemical materials. More particularly, this invention relates to a controlled delivery system for biologically active substances and other chemical materials. Still more particularly, this invention relates to a polymeric system for controlled release in which the release of materials is capable of being switched on or off by adjustment of temperature above or below the glass transition temperature of the polymeric material.
2. Description of the Prior Art
R. F. Stewart (U.S. Pat. No. 4,830,355) has utilized the phenomenon of crystalline melting transition (Tm) in a side chain crystallizable polymer to accomplish the temperature activated controlled released of active agents. At temperatures below the melt temperature of the polymer, the polymer is effectively non-permeable and it acts as a membrane barrier with extremely low diffusional rates for most active agents. In this non-permeable state, the polymer has the ability to protect and contain the desired agent. Above the Tm, the polymer is highly permeable with correspondingly high diffusional rates. The temperature activated release occurs when the polymeric device is heated above its Tm.
T. Okano and coworkers [J. of Contr. Rel. 11, p. 255 (1990)] have described on-off switching polymers for drug permeation and release controlled by temperature change. In this system, water swollen cross-linked interpenetrating network of poly(N-isopropylacrylamide) and polyurethane is used as the drug matrix. A decrease in temperature of the gel results in higher drug release rates due to "gel squeezing effect".
R. P. Sweet, et al. (U.S. Pat. No. 4,840,796) describe a copolymer drug delivery system having soft and hard segments. The soft segment is polydiorganosiloxane having a low Tg of about -125.degree. C. while the hard segment has a high Tg. The copolymer matrix is therefore drug permeable at all practical temperatures, i.e, above -125.degree. C.